1. Field of the Invention
The present invention generally relates to optical semiconductor devices and methods for controlling the same, and more particularly, to an optical semiconductor device having an optical semiconductor element in which a semiconductor laser diode and an optical modulator are integrated and a method for controlling such an optical semiconductor device.
2. Description of the Related Art
In optical communications, an optical signal modulated at high speed is transmitted over an optical fiber. Generally, the optical signal may be modulated by one of the following two methods. The first method directly modulates the laser diode. The second method modulates the optical output signal of the laser diode. The second method uses an optical modulator together with the laser diode. In the past, the laser diode and the optical modulator were discrete components and were separately packaged. Recently, the laser diode and the optical modulator are integrated on a single semiconductor substrate. This kind of laser diode is called MI-LD (Modulator Integrated Laser Diode).
In MI-LD, a pn junction of the optical modulator and another pn junction of the laser diode are formed on the semiconductor substrate. Due to limitations resulting from crystal growth, the two pn junctions on the semiconductor substrate are formed in the same direction and have the same polarity. In operation, the pn junction of the laser diode is forward-biased, while the pn junction of the optical modulator is reverse-biased. Therefore, two power supplies having the opposite polarities are needed to generate the respective bias voltages. One of the two power supplies is a positive supply, and the other is a negative supply.
However, the use of two power supplies needs a large-scale circuit for driving the MI-LD.